This invention relates, in general, to surface acoustic wave (SAW) devices, and more specifically, to SAW transducers.
SAW devices are increasingly operating at higher frequencies. These higher frequencies demand increasingly higher line resolution in the transducer and filter electrodes.
An optimum width for SAW electrodes is one-quarter (1/4) wavelength. This width, however, results in reflections of the generated energy. Specifically, as energy encounters an electrode edge, a reflection results at center frequency. As the energy continues through the electrode and encounters the opposite edge, an additional reflection results. This reflected energy combines with other reflected energy and results in major distortion to the SAW.
Most of the present SAW device electrodes are designed with one-eighth (1/8) or one-sixth (1/6) wavelength widths since these widths eliminate center frequency reflections. Using the smaller wavelength widths decreases the line resolution of the electrodes. Furthermore, the smaller the wavelength, the smaller the electrode must be. For instance, at one GHz, a one-quarter wavelength electrode is approximately 0.9 microns, while a one-eighth wavelength is approximately 0.45 microns. Also, the width of the electrode is inversely proportional to the frequency. The one-quarter wavelength electrode facilitates twice the SAW operating frequency as the one-eighth wavelength electrode geometry for a given line width resolution.
One approach to eliminating quarter wavelength acoustic center frequency reflections is presented in an article entitled "IIDT Type SAW Filter Using Acoustic Reflection Cancel Condition With Solid IDT" by Mishima et al., 1989 IEEE Ultrasonics Symposium, which has not yet been published. This approach has a very specific use and is limited to reflections on 128.degree. LiNbO3 SAW devices used in cellular radio filters.